itch.io

Unassigning trainers doesn't seem to have a consequence other than not being to make them do a job if they don't have a trainer when you go to the next turn, so it would probably solve potential exploits the most cleanly if you set it to trainers having the ability to only do as many trainings per day as they have training slots.  It's technically possible to unassign someone from training, switch them over to another potential trainer just to "hold" the character, then have the same "best" trainer (one with all the bonuses to training, probably the master,) do all the actual training while just fobbing off the character to a holding "trainer" between days.  That's probably more effort than most people want to go through, but you probably wouldn't want to mechanically reward cheese like that, anyway.